1. Related Application
This application is related to U.S. patent application Ser. No. 10/939,674 entitled “Apodized Diffraction Grating With Improved Dynamic Range” of Kenneth R. Wildnauer, which is filed on the same day as this application and is assigned to the assignee of this application.
2. Discussion of Related Art
Modem research and technology have created major changes in the lives of many people. A significant example of this is fiber optic communication. Over the last two decades, optical fiber lines have taken over and transformed the long distance telephone industry. Optical fibers also play a dominant role in making the Internet available around the world. When optical fiber replaces copper wire for long distance calls and Internet traffic, costs are dramatically lowered and the rate at which information can be conveyed is increased.
To maximize bandwidth, that is, the rate at which information can be transmitted, it is generally preferable for multiple information streams to be conveyed over the same optical fiber using multiple optical signals. Each optical signal is a light beam having a wavelength that is unique among the optical signals that share the optical fiber. Optical communication systems rely on optical devices that operate with single wavelength light beams that include a single optical signal, and with multi-wavelength light beams that include multiple optical signals. Such optical devices include, among others, spectral filters.
A spectral filter receives an input light beam that includes two or more spectral components that have different wavelengths. The filter selects and outputs only those input beam components that have wavelengths within a narrow range. This range is determined by the characteristics of the spectral filter. The center of the range defines the wavelength of the spectral filter.
It is desirable for a spectral filter to separate light beam components having only small wavelength differences. Small wavelength differences in light beam components are desirable because, for example, the conventional or “C” optical communication band can only support up to about 40 independent optical signals that are separated in wavelength by an increment of about 200 gigahertz (GHz). However, if the optical communication system can support wavelengths that differ by only about 25 GHz, then the “C” band can support over 150 independent signals.
Light beam components separated by only small wavelength increments can be combined into a densely packed multi-wavelength beam. Using such a light beam enables an optical communication system to convey a large amount of information over a single optical fiber. However, such dense packing requires precise combination, separation, and other handling of these light beams.
FIG. 1A is a graph showing a multi-wavelength beam superimposed on the transmission spectrum of an exemplary spectral filter. Transmission spectrum 110 is graphed as the logarithm of the intensity of the light transmitted by the spectral filter, with respect to the wavelength of the light.
The multi-wavelength light beam depicted in FIG. 1A has two light beam components 120 and 130. Each light beam component 120 and 130 has a single wavelength, and each component is graphed as the logarithm of the intensity of the component at the particular wavelength of the component. Notably, the intensity of component 120 is substantially lower than component 130.
Transmission spectrum 110 includes a single primary peak 112 and a number of side lobes 114. The center of primary peak 112 is the spectral filter wavelength. The wavelength of light beam component 120 coincides with the spectral filter wavelength. Primary peak 112 allows light beams of the spectral filter wavelength to pass through the spectral filter without a substantial decrease in intensity.
Side lobes 114 within transmission spectrum 110 are shown occurring in a periodic pattern as the wavelength of the light varies. Side lobes 114 decrease in intensity as the wavelength difference increases between a particular side lobe and primary peak 112. The wavelength of component 130 is shown coinciding with one of the stronger side lobes.
Side lobes 114 may affect system performance since they allow light at undesired wavelengths to pass through the spectral filter. For example, consider the scenario of the multi-wavelength beam shown in FIG. 1A, denoted by components 120 and 130, which are communicated to a spectral filter having the transmission spectrum shown in FIG. 1A. The desired output from the spectral filter would be all of light beam component 120, while all of light beam component 130 is blocked. However, as shown in FIG. 1B, this not always possible.
FIG. 1B is a graph showing the intensity of the light transmitted by a spectral filter having the transmission spectrum shown in FIG. 1A. The intensity of light beam components 121 and 131 represent the output that would be provided by the spectral filter.
Output component 121 typically has about the same intensity as input component 120 since the wavelength of the spectral filter transmits substantially all of the input light at that particular wavelength. However, output component 131 has a much lower intensity than input component 130 because the spectral filter substantially attenuates light at the wavelength of this component. Output component 131 has a somewhat higher intensity than output component 121 because input component 130 has a substantially higher intensity than input component 120. In general, the spectral filter described in FIG. 1A cannot readily be used with a multi-wavelength input light beam since it is difficult or impossible to detect input component 121 because of interference from component 131. In addition, many conventional spectral filters have limited transmission width and rejection shape, which fall below the requirements of modern optical communication systems.